The present invention relates to a device for drawing a fiber, particularly an optical glass fiber, from a heated preform, the apparatus having a tubular heating element which encloses the preform at least partially in its longitudinal direction, and an induction coil surrounding the tubular heating element at a radial distance.
Due to the small outside diameters of optical glass fibers, direct production of the required fiber profile from different glass types is not possible. The basis for producing an optical glass fiber is therefore a so-called preform with a diameter in the centimeter range, which may be produced by a wide variety of processes.
Glass is an amorphous material that can be brought to a state of low viscosity by heating. In quartz glasses, which are typically used for fiber production today, the glass noticeably softens only at temperatures of approximately 2000 to 2200xc2x0 C. This softening of the glass at a corresponding temperature is used to draw out the preform into a thin glass fiber. For this purpose, the preform is heated in a so-called drawing oven.
DE 43 39 077 C2 describes such a drawing oven, which has a tubular heating element of graphite and an annular induction coil surrounding this tubular heating element at a radial distance. Due to the high temperatures of the heating element and the oxygen present in the oven chamber, which is purged with a protective gas, graphite particles may be released from the hot surface of the heating element made of graphite, and may accumulate on the surface of the glass fiber. Furthermore, this release of graphite particles causes the surface of the heating element to burn off and thus shortens the life of the heating element.
An object of the present invention is thus to prolong the service life of the heating element of a drawing oven, prevent the release of particles from the hot surface of the heating element and thus reliably prevent any contamination of the drawn fiber surface by the particles released from the heating element.
The invention attains this object by providing a protective tube along the inside of the tubular heating element enclosing the fiber and the preform at a radial distance.
The advantages that may be obtained by the invention are in particular that the protective tube prevents the tubular heating element from burning off along its hot inner surface since the protective tube partitions off the heating element from the oxygen present in the oven chamber. This significantly prolongs the service life of the heating element and significantly improves the quality of the fibers drawn from the preform due to the absence of particle emissions from the heating element.
Additional features and advantages of the invention will be apparent from the description below and the appended claims.
For a particularly effective partitioning of the heating element from the oxygen present in the oven chamber, it is advantageous if the protective tube is oxygen impermeable.
It is particularly advantageous if the protective tube is made of glassy carbon. Glassy carbon is highly heat resistant and very durable. Isotropic bodies of glassy carbon have a high-quality surface finish, a dense structure, high chemical purity and good chemical resistance. A protective tube made of glassy carbon furthermore has properties similar to those of the heating element made of graphite.
It is also advantageous if the protective tube is made of mesophase graphite, which prevents the emission of particles from the surface of the heating element and itself does not emit any particles at the elevated temperatures in the drawing oven.
For the same reasons, it is also advantageous if the protective tube is made of a ceramic material such as silicon carbide or silicon nitride. These materials have electrical and physical properties similar to those of graphite, which is frequently used for the heating element.
To provide a simple structure of the apparatus according to the invention and reliably to prevent the heating element surface from burning off, it is advantageous if the protective tube and the heating element form a unit, particularly if the protective tube is integrated into the heating element.